The present invention relates generally to the problem of positioning of a mobile station (xe2x80x9cMSxe2x80x9d) in a Public Land Mobile Network (xe2x80x9cPLMNxe2x80x9d), and more particularly to the problem of establishing the most accurate estimate of the last known position of the mobile station when the mobile station has switched off or has left the coverage area of the network.
There are currently developing various commercial and regulatory needs to be able to use the Public Land Mobile Network (xe2x80x9cPLMNxe2x80x9d) to geographically locate Mobile Stations (xe2x80x9cMSxe2x80x9d). An example of a PLMN is the GSM (xe2x80x9cGlobal System for Mobile communicationsxe2x80x9d) system. A simplified schematic of a GSM system is shown in FIG. 1a. There is first a mobile station 110 which is usually the only equipment the subscriber sees from the whole system. The subscribers telephone, or terminal, connects over a radio interface to Base Station Sub-System (xe2x80x9cBSSxe2x80x9d) 120. Roughly speaking, the Base Station Sub-System 120 groups together the infrastructure machines which are specific to the radio cellular aspects of GSM.
The BSS 120 is in direct contact with mobile stations through the radio interface. On the other side, the BSS 120 is in contact with the switches of the Switching System (xe2x80x9cSSxe2x80x9d) 130. The role of the BSS 120 can be summarised as to connect the mobile station 110 and the Switching System 130, and hence the mobile station""s user with other telecommunications users. The BSS 120 has to be controlled and is thus also in contact with the Operation Sub-System (xe2x80x9cOSSxe2x80x9d) 140.
The Switching Sub-system 130 includes the main switching functions of GSM, as well as the databases need for subscriber data and mobility management. The main role of the Switching Sub-system is to manage the communications between the GSM users and other telecommunications network users. These other networks can include other PLMNs 151, the PSTN (xe2x80x9cPublic Switched Telephone Networkxe2x80x9d) 152, CSPDN (xe2x80x9cCircuit Switched Public Data Networkxe2x80x9d) 153, PSPDN (xe2x80x9cPacket Switched Public Data Networkxe2x80x9d) 154, and ISDN (xe2x80x9cIntegrated Services Digital Networkxe2x80x9d) 155.
Within the SS, the basic switching function is performed by the Mobile Services Switching Centre (xe2x80x9cMSCxe2x80x9d) 131, whose main function is to co-ordinate the setting-up of calls to and from GSM users. The MSC 131 has interfaces with the BSS 120 on one side and with the external networks 150 on the other side. However, the direct connections between the external networks 150 and a GSM system are routed through a Gateway-MSC (xe2x80x9cGMSCxe2x80x9d), not shown here. All incoming calls for a GSM/PLMN network will be routed to one or more GMSCs which work as an incoming transit exchange for the GSM/PLMN. Besides MSCs 131, the Switching Sub-system 130 includes databases. Subscriber information relevant to the provision of telecommunications services is held on the infrastructure side in the Home Location Register (xe2x80x9cHLRxe2x80x9d) 132, independently of the actual location of the subscriber. The HLR 132 also includes some information related to the current location of the subscriber. Every telecommunications system includes a database containing a variety of information concerning each subscriber, such as the subscription limitations, the services subscribed to, charging information, etc. In a fixed network, each subscriber is connected to one local switch, for a long time. Every call involving this subscriber goes through this switch. This is then the natural place to store the subscriber related information. In a system dealing with moving subscribers, there is no such natural place for storage. However, two kinds of data to be stored (location information and subscriber data) call for a common storage solution. This is the choice made in GSM, and the HLR 132 is the database for both sets of information.
If location information is needed only for the establishment of mobile terminating calls, the rest of the information is needed at various moments during any call. Basically, it is the visited Mobile Switching Centre 131, the one in charge of a mobile subscriber engaged in a call, which needs these pieces of information. This would result in a substantial signalling load if the MSC 131 had to interrogate the HLR each time it needs some piece of information.
To avoid this signalling load, the data record of a subscriber is copied into a database close to the MSC 131 while this subscriber is registered in a location area (xe2x80x9cLAxe2x80x9d) controlled by the MSC 131. This database is the Visitor Location Register (xe2x80x9cVLRxe2x80x9d) 133. The information regarding those subscribers which are registered under an MSC 131 connected to the VLR 133 is stored in the VLR 133, but only temporarily.
This introduces new functions. The subscriber information has to be copied when the subscriber enters a new MSC/VLR area. Conversely, the corresponding record has to be erased in the previous MSC/VLR area in which the subscriber was registered. This updating of the databases is done when certain events occur. These events include call set-up, handovers, call complete, IMSI (International Mobile Subscriber Identity) attach (i.e. when the mobile station switches on), IMSI detach (i.e. when the mobile station switches off) and location updates.
As mentioned above, there is increasing demand for the development of PLMN-based positioning. PLMN-based positioning can be considered to consist of four parts: a Positioning Procedure, a positioning gateway, applications, and signalling mechanisms.
The first of these four parts is a Positioning Procedure. This is a mechanism which is used by the network, e.g. GSM, to find out and report relative, or in some cases absolute, data about the location of the Mobile Station. Different Positioning Procedures can exist in the network simultaneously. These different Positioning Procedures can each have their own technical solutions and degrees of accuracy. The degrees of accuracy can range from the whole Service Area of the PLMN down to below 100 meters. However, in general, the higher the degree of accuracy, the higher the cost to the network in terms of signalling capacity and load.
One example of such a Positioning Procedure is GPS, Global Positioning System. GPS is satellite based system wherein a mobile station must be equipped with a GPS terminal. The GPS terminal can determine its position through reception of signals from at least three satellites. The position of the mobile station can then be acquired by the application, for example, polling the GPS terminal, after which the position information is sent over the network to the application.
Another possible Positioning Procedure is for the PLMN to measure the wave propagation time for the signal from the mobile station to a base station. The propagation time then corresponds to the position of the mobile station. The position of the MS can then be determined with certainty by repeating these measurements from two more base stations. It is also known to determine the mobile stations position by having the mobile station perform measurements on the signals from three base stations instead.
The second part of PLMN-based positioning is a positioning gateway. In the present patent application this will be referred to as a Mobile Positioning Centre (xe2x80x9cMPCxe2x80x9d) 170. The Mobile Positioning Centre uses the data from the Positioning Procedure, e.g. GPS, to calculate the absolute position of the Mobile Station and distribute the position information to the applications, discussed below. The MPC 170 is connected to the PLMN via the Gateway Mobile services Switching Centre (xe2x80x9cGMSCxe2x80x9d). An example of a possible system using a Mobile Positioning Centre 170 is shown in FIG. 1b. 
The third part of PLMN-based positioning consists of the applications that are the users of the positioning information retrieved from the network. The applications send the request for positioning information to the Mobile Positioning Centre 170, which retrieves the information from the network in a method according to the present invention and then forwards this positioning information onto the application. Examples of applications can include: fleet management, emergency services such as ambulance, and location dependent services like the nearest fuel facility or perhaps stolen vehicle recovery, etc.
The fourth, and final, part of a PLMN-based positioning system are the signalling mechanisms of the network. These are used for transferring the positioning data between the various elements used for determining the positioning of a mobile station.
Positioning systems based on the infrastructure and radio interface in a Public Land Mobile Network are not yet on the market. The only location information available in the system today when the mobile station is in xe2x80x9cidle modexe2x80x9d is not very accurate. Idle mode is the mode of the mobile station when it merely listens to broadcast channels without having a channel of its own. It contrasts with xe2x80x9cdedicated modexe2x80x9d where a bi-directional channel is allocated to the mobile station for its communication needs, allowing it to exchange point-to-point information with the infrastructure in both directions.
The best known area in GSM today when the MS is in idle mode is the Location Area (xe2x80x9cLAxe2x80x9d), which could be more than 100 kilometres wide. In future positioning systems there will be several ways to discover the location of a Mobile Station by using different Positioning Procedures.
The Positioning Procedure could be terminal-based or network-based. In a terminal-based solution it is the mobile stations that find out and report relative positioning data to the Mobile Positioning Centre for calculation of the absolute position. In a network-based solution it is the Base Station System 110 that reports the relative data to the Mobile Positioning Centre 170. A Positioning Procedure could also be a combination of both a terminal-based and a network-based procedure. Other Positioning Procedures could consist of external positioning equipment connected to the Mobile Station, e.g. GPS.
A problem that exists with these Positioning Procedures is based on the need to be able to locate the MS with an accuracy good enough for the applications. This need requires the network to be able to establish contact with the Mobile Station. This means that the Mobile Station must be switched on and within the coverage area of the network.
However a problem occurs when the Mobile Station is switched off or has left the coverage area of the network. It is then impossible to get accurate information about where and when the last known position of the Mobile Station was. However, in these circumstances the best information available in GSM, for example, is the Location Area. This accuracy is not good enough for most of the applications and there is no mechanism to guarantee that the information is available in the system when desired by the application.
The solution to the above described problem is to update the subscriber databases available in the system on a regular basis with accurate positioning data. The data can then be used to obtain the last known location of the Mobile Station when it can""t be reached. What positioning data that is to be stored is a question of implementation. This could be, for example, the cell identification, or longitude and latitude as is used in GPS. However, since data is to be stored for all Mobile Stations moving around in the network, it is important to use a system friendly implementation that doesn""t cause too much extra load which steals capacity in the network, but which still gives an accuracy that is good enough for the applications.
In the preferred embodiment of the present invention as implemented in GSM, there are two subscriber databases that will be updated. These are the Visitor Location Register 133 and the Home Location Register 132. The Visitor Location Register 133 is used for temporary storage while the Home Location Register 132 is used for permanent storage.
The Visitor Location Register 133 is updated when certain events occur that make the position information available to the system. Since the VLR 133 is only used for temporary storage of subscriber data, the positioning information must be transferred to the HLR 132 for permanent storage when the subscriber record in the VLR 133 is to be deleted for some reason.
An application first sends a request to the Mobile Positioning Centre, requesting position information for a particular subscriber. The MPC contacts the preferred Positioning Procedure to obtain the positioning information. If this information is not available then the MPC contacts the HLR to get the last known positioning data. The HLR will try to retrieve the last known positioning data from the VLR and forward it to the MPC. If this is not available then the last known position stored in the HLR is forwarded to the MPC.
The result of the present invention is that it makes it possible to obtain the last known location of a mobile station with good accuracy without stealing air interface or when the mobile station can""t be reached by the normal Positioning Procedures. This increases the value of positioning for many applications.
It also makes it possible for the PLMN operator to offer xe2x80x9coldxe2x80x9d positioning data at a lower price since the cost of obtaining the position is lower when compared to other xe2x80x9creal-timexe2x80x9d Positioning Procedures, since no contact needs to be established with the mobile station.
Another benefit of the present invention is that the saved data can improve paging of mobile stations since the network can start paging in a specific cell instead of a whole Location Area. This will, of course, decrease the signalling in the network in most cases.